Various imaging techniques have been employed for detecting and locating cancerous tumors in body tissue. X-ray and ultrasound imaging techniques are commonly utilized in screening for breast cancer. X-ray mammography is the most effective current method for detecting early stage breast cancer. However, X-ray mammography suffers from relatively high false positive and false negative rates, requires painful breast compression, and exposes the patient to low levels of ionizing radiation.
Microwave based imaging methods have been proposed for use in imaging of breast tissue and other body tissues as an alternative to current ultrasound and X-ray imaging techniques. Microwave imaging does not require breast compression, does not expose the patient to ionizing radiation, and can be applied at low power levels. Microwave-based imaging exploits the contrast in dielectric properties between normal and malignant tissue. With microwave tomography, the dielectric-properties profile of an object being imaged is recovered from measurement of the transmission of microwave energy through the object. This approach requires the solution of an ill-conditioned nonlinear inverse-scattering problem which requires elaborate image reconstruction algorithms. An alternative microwave imaging approach is based on microwave radar methods that use the measured scattered signal to infer the locations of significant sources of scattering in the object being imaged, and are simpler to implement and more robust. Microwave radar methods require the focusing of the received signal in both space and time to discriminate against clutter and to obtain acceptable resolution. This may be accomplished with an antenna array and ultra-wideband microwave probe signals. For a discussion of this approach, see, S. C. Hagness, et al., “Two-Dimensional FDTD Analysis of a Pulsed Microwave Confocal System for Breast Cancer Detection: Fixed Focus and Antenna-Array Sensors,” IEEE Trans. Biomed. Eng., Vol. 45, Dec., 1998, pp. 1470-1479; S. C. Hagness, et al., “Three-Dimensional FDTD Analysis of a Pulsed Microwave Confocal System for Breast Cancer Detection: Design of an Antenna-Array Element,” IEEE Trans. Antennas and Propagation, Vol. 47, May, 1999, pp. 783-791; S. C. Hagness, et al., “Dielectric Characterization of Human Breast Tissue and Breast Cancer Detection Algorithms for Confocal Microwave Imaging,” Proc. of the 2nd World Congress on Microwave and Radio Frequency Processing, Orlando, Fla., April, 2000; X. Li and S. C. Hagness, “A Confocal Microwave Imaging Algorithm for Breast Cancer Detection,” IEEE Microwave and Wireless Components Letters, Vol.11, No. 3, March, 2001, pp.130-132; and E. Fear, et al, “Confocal microwave imaging for breast cancer detection: Localization of tumors in three dimensions,” IEEE Transactions on Biomedical Engineering, vol. 49, no. 8, August 2002, pp. 812-822.
This approach has been extended using space-time beamforming. E. J. Bond, et al., “Microwave Imaging Via Space-Time Beamforming for Early Detection of Breast Cancer,” IEEE Trans. Antennas and Propagation, Vol. 51, No. 8, August 2003, pp.1690-1705; S. K. Davis, et al, “Microwave imaging via space-time beamforming for early detection of breast cancer: Beamformer design in the frequency domain,” Journal of Electromagnetic Waves and Applications, vol. 17, no. 2, 2003, pp. 357-381; and X. Li, et al, “Microwave imaging via space-time beamforming: Experimental investigation of tumor detection in multi-layer breast phantoms,” IEEE Transactions on Microwave Theory and Techniques, vol. 52, no. 8, August 2004, pp.1856-1865. See also U.S. published patent application 2003/0088180 A1, “Space-Time Microwave Imaging for Cancer Detection,” published May 8, 2003, the disclosure of which is incorporated by reference.